Development of a vascular supply is a fundamental requirement for many physiological and pathological processes. For example, the process of angiogenesis, which promotes new blood vessel formation, ensures that actively growing tissues get an adequate blood supply. Angiogenesis, however, also is implicated in the pathogenesis of a variety of disorders. The process of vascular development, including endothelial cell differentiation, proliferation and migration is tightly regulated. For example, members of the Notch-signaling pathway play a role in regulating processes such as cell fate determination, cellular differentiation, proliferation, survival and apoptosis, including in the vascular endothelium. The Notch ligand, Delta-like 4 (DLL4), exhibits highly selective expression in the vascular endothelium. Notch signaling also is involved in a wide variety of human diseases. Thus, there is a need to develop agents that can be used as therapeutics to to regulate Notch signaling.